Many chemical separation analyses, such as gas and liquid chromatography, require the chemical sample to be temperature-controlled throughout the analysis.
A chromatograph comprises an inlet where the sample is introduced, an oven containing an analytical column where the separation takes place, and a detector where the constituents of the sample are detected and recorded. Each of these parts of the instrument is temperature-controlled to ensure the integrity and repeatability of the analysis. An analysis performed at a constant controlled temperature is referred to as isothermal. To perform an isothermal analysis, the analytical column is typically placed in a temperature-controlled chamber, often referred to as an oven that is preheated to the desired temperature. A non-isothermal analysis, where the column temperature is gradually raised over time, is also common, especially for samples with relatively massive components that would otherwise take a long time to elute from the column.
Conventional chromatographic ovens typically use convection technology to heat and maintain the interior of the chamber, and hence the column, at the desired temperature. However, conventional ovens are relatively large in comparison to an analytical column which they are intended to heat and, as a result, are very power inefficient. In addition to cost, a side effect of power inefficiency is that the oven is slow to heat and cool, resulting in reduced sample throughput and productivity.
One prior solution to reduce power consumption when heating an analytical column is to use a resistively heated analytical column. Unfortunately, this technology requires a specially fabricated column that may be incompatible with existing chromatography systems. In addition, an analytical column is susceptible to contamination at its input usually due to sample build-up over time. The contaminated portion of the analytical column is typically removed so the column can be reused. This is difficult or impossible to do when using a resistively heated column since the column and heating element are bundled together. Further, it is difficult to precisely determine the temperature of a resistively heated analytical column because it is difficult to place a temperature probe so that its temperature tracks the temperature of the resistively heated column precisely.
Another prior solution to reduce power consumption when heating an analytical column is to use an electromagnetic (EM) radiant source, such as a microwave or infrared source. Unfortunately, capillary columns, which represent the overwhelming majority of analytical columns used in gas chromatographic analyses today, are typically fabricated from fused silica glass, which is transparent to radiant energy. To take advantage of being heated by radiant energy the analytical column must be coated, or otherwise treated, with a material or substance that can absorb the radiation emitted from the radiant source and convert the radiant energy to heat. Also, as with the resistively heated analytical column, determining the precise temperature of an analytical column heated by a radiant source is difficult because it is difficult to ensure that a temperature probe absorbs and converts the radiant energy to heat in the same way as the column to provide an accurate measure of the column temperature. Finally, the directional or “line-of-sight” nature of an EM radiant source adds a potential source of temperature gradients across the column that would not be present in a conventional convection oven.
Therefore, it would be desirable to efficiently heat a conventional analytical column and accurately determine its temperature.